Conventionally, for a lithium secondary battery, LiCoO2 is mainly used as a positive active material. However, the discharge capacity is about 120 to 130 mAh/g.
Materials obtained by forming solid solutions of LiCoO2 with other compounds are known. A solid solution, Li[Co1−2xNixMnx]O2 (0<x≦1/2), having an α-NaFeO2 type crystal structure and including three components, LiCoO2, LiNiO2, and LiMnO2, was disclosed in 2001. A lithium secondary battery using one example of the above-mentioned solid solutions, LiNi1/2Mn1/2O2 or LiCo1/3Ni1/3Mn1/3O2, as an active material has a discharge capacity of 150 to 180 mAh/g and is thus more excellent than that using LiCoO2.
Non-Patent Documents 1 to 4 have proposed a solid solution having an α-NaFeO2 type crystal structure and including three components, Li[Li1/3Mn2/3]O2, LiNi1/2Mn1/2O2 and LiCoO2. The material is expressed as Li[Li, Mn, Ni, Co]O2. That is, this material is a LiCoO2 material having an α-NaFeO2 type crystal structure and including a transition metal and Li in the site where Co exists. Therefore, it is expected that the discharge capacity is further increased. Non-Patent Documents 1 to 4 report that the discharge capacity becomes about 180 to 200 mAh/g.
However, an active material for a lithium secondary battery with furthermore improved discharge capacity has been desired.
Many trials of substitution a part of the transition metal site of a transition metal compound to be used for a positive active material for a lithium secondary battery with a different kind of element have been investigated, including, no need to say, an example of other active materials such as LiMn2O4 with a tetragonal spinel structure. However, the effect of the substitution with a different kind of element differs depending on each active material. Therefore, in this technical field, whether the effect caused in the case of one different material can be also caused in the case of another material is very difficult to expect and that is beyond discussion.
Non-Patent Document 5 discloses that as a result of substitution of a part of Co of LiCoO2 with Mg, although the electron conductivity at room temperature is improved (see FIG. 2), but the discharge capacity is lowered by Mg addition (see FIG. 6 and FIG. 8).
Non-Patent Document 6 discloses a result of substitution of a part of a transition metal site of LiCo1/3Ni1/3Mn1/3O2, which is equivalent to a solid solution including three components, LiCoO2, LiNiO2, and LiMnO2, with Mg. Owing to the substitution, similarly, the discharge capacity is lowered (see FIG. 8).
Non-Patent Document 7 discloses a result of substitution of a part of a transition metal site of Li[Li0.15Ni0.275Mn0.575]O2, which is equivalent to a solid solution including two components, Li[Li1/3Mn2/3]O2 and LiNi1/2Mn1/2O2, with Mg. Owing to the substitution, the capacity retention ratio along with repeated charge-discharge is improved. However, the initial discharge capacity is similarly lowered (see FIG. 2). Further, the reported discharge capacity does not exceed 200 mAh/g (see the same drawing).
Further, the invention disclosed in claim 6 of Patent Document 1 is “a positive active material comprising a composite oxide, as a main component, represented by Li[MncNidCoeLiaM″b]O2 (M″ is at least one element selected from the group consisting of B, Mg, Al, Ti, V, Cr, Fe, Cu, and Zn; d≦c+e+a+b; c+d+e+a+b=1; 0≦a≦0.05; 0≦b≦0.05; 0.2≦c≦0.5; 0.02≦e≦0.4), wherein the positive active material has a specific surface area of 0.3 m2/g or more and 1.5 m2/g or less, an X-ray diffraction pattern capable of belonging to space group R3/m, a relative intensity ratio of the diffraction peak at 2θ=44.1±1° to the diffraction peak at 2θ=18.6±1° of 0.6 or more and 1.1 or less, the half width of the diffraction peak at 2θ=18.6±1° of 0.13° or more and 0.20° or less, the half width of the diffraction peak at 2θ=44.1±1° of 0.10° or more and 0.17° or less, and a particle diameter of 3 nm or more and 20 nm or less”. In this document, it is also described that a nonaqueous electrolyte secondary battery provided with both satisfactory high rate discharge performance and satisfactory charge-discharge cycle performance can be obtained by using a positive active material with the relative intensity ratio adjusted to 0.6 or more and 1.1 or less. However, it is not described that in the case where the relative intensity ratio of an active material having a specified composition in which Mg is employed for M″ is within a specific range, the discharge capacity is significantly improved and that in the case where the half width of the active material having a specified composition in which Mg is employed for M″ is within a specified range, the high rate discharge characteristics are significantly improved.
The invention disclosed in claim 1 of Patent Document 2 is “a positive active material comprising a composite oxide represented by the composition formula LiaMn0.5−xNi0.5−yMx+yO2 (0<a<1.3; −0.1≦x−y≦0.1; M is an element other than Li, Mn, and Ni)”. Further, from the 7th line from the bottom of page 6 to the 4th line of page 7 in this document, it is described that “the positive active material according to 2 has a feature of containing a composite oxide in which M is at least one element selected from the group consisting of Al, Mg, and Co and the coefficients in the composition formula satisfy the following relational expressions, 0.05≦x<0.3; 0.05≦y<0.3; −0.1≦x−y≦0.02; 0<a<1.3; and x+y<0.5. Owing to such a configuration, it is made possible to particularly form a positive active material which can provide a nonaqueous electrolyte secondary battery excellent in high rate discharge performance and charge-discharge cycle performance and having a high energy density”. Further, from the 4th line from the bottom of page 8 to the 3rd line of page 9 in this document, it is described that “the positive active material according to 5 has a feature that the composite oxide . . . has the following total fine pore volume and that the relative intensity ratio of the diffraction peak at 2θ=44.1±1° to the diffraction peak at 2θ=18.6±1° in the powder X-ray diffraction diagram using CuKα-ray is 0.65 or more and 1.05 or less. Owing to such a configuration, it is made possible to particularly form a positive active material which can provide a nonaqueous electrolyte secondary battery excellent in high rate discharge performance and charge-discharge cycle performance and having a high energy density (high discharge capacity)”. Furthermore, from the 4th line to the 10th line of page 9 in this document, it is described that “the positive active material according to 6 has a feature that the composite oxide . . . has the following specific surface area and that the relative intensity ratio of the diffraction peak at 2θ=44.1±1° to the diffraction peak at 2θ=18.6±1° in the powder X-ray diffraction diagram using CuKα-ray is 0.65 or more and 1.05 or less. Owing to such a configuration, it is made possible to particularly form a positive active material which can provide a nonaqueous electrolyte secondary battery excellent in high rate discharge performance and charge-discharge cycle performance and having a high energy density (high discharge capacity)”. However, it is not described that in the case where the relative intensity ratio of an active material having a specified composition in which Mg is employed for M is within a specified range, the discharge capacity is improved significantly”.
Also, from the 11th line to 16th line of page 9 of Patent Document 2, it is described that “the positive active material according to 7 has a feature that the half width of the diffraction peak at 2θ=18.6±1° is 0.05° or more and 0.20° or less and the half width of the diffraction peak at 2θ=44.1±1° is 0.10° or more and 0.20° or less. Owing to such a configuration, it is made possible to particularly form a positive active material which can provide a nonaqueous electrolyte secondary battery having a high energy density (high discharge capacity) and excellent in charge-discharge cycle performance“. However, it is not described in this document that in the case where the half width of the diffraction peak of an active material having a specified composition in which Mg is employed for M is within a specified range, the high rate discharge characteristics are improved significantly“.
The invention disclosed in claim 1 of Patent Document 3 is ”an active material for a lithium secondary battery comprising a solid solution of a lithium-transition metal composite oxide having an α-NaFeO2 type crystal structure, wherein the composition ratios of Li, Co, Ni, and Mn contained in the solid solution satisfy Li1+1/3xCo1−x−yNiy/2Mn2x/3+y/2 (x+y≦1; 0≦y; 1−x−y=z) and in a ternary phase diagram of Li[Li1/3Mn2/3]O2 (x)-LiNi1/2Mn1/2O2 (y)-LiCoO2 (z), (x, y, z) is represented by values existing on lines of or within a heptagon ABCDEFG defined by vertexes; point A (0.45, 0.55, 0), point B (0.63, 0.37, 0), point C (0.7, 0.25, 0.05), point D (0.67, 0.18, 0.15), point E (0.75, 0, 0.25), point F (0.55, 0, 0.45), and point (0.45, 0.2, 0.35) and the intensity ratio between the diffraction peak of (003) plane and the diffraction peak of (104) plane measured by X-ray diffractometry is I(003)/I(104)≧1.56 before charge-discharge and I(003)/I(104)>1 at the end of discharge”. Further, the invention disclosed in claim 10 of Patent Document 3 is “a method for producing the lithium secondary battery according to claim 9 employing a charging method where the positive electrode upon charge has a maximum reaching potential of 4.3 V (vs. Li/Li+) or less, the method comprising the step of charging in such a manner that the charge attains at least a region appearing in a positive electrode potential range exceeding 4.3 V (vs. Li/Li+) and 4.8 V (vs. Li/Li+) or less and having a relatively plateau potential variation”. Further, in the paragraph 68 of this document, it is described that “in a conventional active material, it is supposed that such disorder phase is formed to inhibit smooth transfer of Li ion and it affects the reversible capacity. On the other hand, in the active material of the present invention, it is supposed that since I(003)/I(104)≧1.56, formation of the disorder phase is extremely slight and thus an excellent discharge capacity can be obtained”. Furthermore, as examples, it is disclosed an active material for a lithium secondary battery which has I(003)/I(104)=1.77 as an intensity ratio between the diffraction peak of (003) plane and the diffraction peak of (104) plane measured by X-ray diffractometry before charge-discharge and I(003)/I(104)=1.67 at the end of discharge and a discharge capacity of 225 mAh/g. However, this active material does not contain Mg. Accordingly, this document does not indicate that the discharge capacity is significantly improved in the case where the relative intensity ratio of an active material having a specified composition containing Mg is within a specified range.
Further, the lithium secondary battery using a solid solution as an active material disclosed in Patent Document 3 has a problem that it is impossible to obtain capacity at the time of high rate discharge as disclosed in comparative examples described below.
The invention disclosed in claim 1 of Patent Document 4 is “a lithium-nickel-manganese-cobalt composite oxide comprising a layer structure and the chemical composition, LiaNixMnyCozO2+b (x+y+z=1, 1.00<a<1.3, 0≦b<0.3), wherein the diffraction peak angle 2θ of (003) plane and the diffraction peak angle 2θ of (104) plane are 18.65° or more and 44.50° or more, respectively, in Miller's index hkl of the powder X-ray diffraction using CuKα-ray, and both the half widths of the diffraction peaks of the planes are 0.18° or less and also the diffraction peak angle 2θ of (108) plane and the diffraction peak angle 2θ of (110) plane are 64.40° or more and 65.15° or more, respectively, and both the half widths of the diffraction peaks of the planes are 0.18° or less. Further, in the paragraph 17, it is described that “if the diffraction peak angle 2θ of (003) plane and the diffraction peak angle 2θ of (104) plane are lower than 18.65° and 44.50°, respectively, in Miller's index hkl of the powder X-ray diffraction using CuKα-ray, the phase interval is decreased and diffusion of lithium ions is inhibited. Therefore, the charge-discharge characteristics are deteriorated. Further, if the half width of the diffraction peak of each of the planes is higher than 0.18°, the crystal growth is insufficient or variation in the composition becomes significant. Therefore, the charge-discharge characteristics are deteriorated”. However, the relation of the half width of the diffraction peak of (003) plane and the half width of the diffraction peak of (104) plane with the high rate discharge characteristics are not described.
The invention disclosed in claim 1 of Patent Document 5 is “a positive active material comprising lithium-containing metal composite oxide particles having a crystal structure belonging to space group R-3m, the half width of the X-ray diffraction peak corresponding to (104) plane in a range of 0.06 to 0.15° and an average value of the shape factor SF1 calculated according to the following expression (1) in a range of more than 1 and 3.3 or less.”. Further, it is described that in the case where the crystal structure of the lithium-containing metal composite oxide particles belongs to space group R-3m and the half width of the X-ray diffraction peak corresponding to (104) plane is within a range of 0.06 to 0.15°, high discharge load characteristics (high rate discharge characteristics) can be obtained. Furthermore, in the paragraph 25 of this document, it is described that if the half width exceeds 0.15°, the crystallinity of the lithium-containing metal composite oxide is lowered and therefore, it becomes difficult to obtain high rate discharge characteristics. However, this document does not at all disclose the addition of Mg to the lithium-containing metal composite oxide. Moreover, it is not also suggested that the high rate discharge characteristics are significantly improved by specifying the range of the half width of the diffraction peak of the lithium-containing metal composite oxide containing Mg.
In the paragraph 77 of Patent Document 6, it is described that “it is made possible to provide an additional function by doping LiNi1/3Mn1/3Co1/3O2 of the present invention with a different kind of element and the addition of magnesium remarkably improves the electron conductivity”. Further, in the paragraphs 28 to 30 of this document, it is also described that an oxide with an increased lithium atomic ratio and represented by Li[Lix(Ni1/3Mn1/3Co1/3)1−x]O2 (wherein, 0≦x≦0.3) can be used. A nickel-manganese-cobalt composite oxide obtained by mixing a composite oxide obtained by coprecipitation and lithium hydroxide in dry state and calcining the mixture at 1000° C. has a hexagonal system belonging to layer structure R3m. However, it is not suggested that if magnesium is added as a solid solution component to a nickel-manganese-cobalt composite oxide, the discharge capacity is significantly improved and the high rate discharge characteristics are remarkably improved.
The invention disclosed in claim 6 of Patent Document 7 is ”a lithium-nickel-manganese-cobalt composite oxide powder for a positive material of a lithium secondary battery comprising a crystal structure belonging to a layer structure and the composition represented by the following formula:Li[Liz/(2+z){(LixNi(1−3x)/2Mn(1+x)/2)(1−y)Coy}2/(2+z)]O2  (I)(wherein 0.01≦x≦0.15; 0≦y≦0.35 and 0.02(1−y)(1−3×)≦z≦0.15(1−y)(1−3×))”. In the paragraphs 14 and 15 of this document, it is described that it is important that the Li amount is in a slightly richer range than that in the stoichiometric composition and that the battery performances (particularly rate characteristics and output characteristics) are increased due to that. However, it is not suggested that in the case where the lithium-nickel-manganese-cobalt composite oxide is a specified composition containing magnesium, the discharge capacity is significantly improved and the high rate discharge characteristics are remarkably improved.
Further, the positive active materials for a lithium secondary battery disclosed in Patent Documents 1 to 7 are not supposed to be a solid solution of four components including Li[Li1/3Mn2/3]O2, LiNi1/2Mn1/2O2, LiCoO2 and LiMg1/2Mn1/2O2. Therefore, even in the case where the above-mentioned positive active materials contain Mg and satisfy the condition of the relative intensity ratio, it cannot be expected that the discharge capacity is improved based on the descriptions of Non-Patent Documents 5 to 7.